The present invention relates to retractable seals for rotary machines such as steam turbines, gas turbines, aircraft engines, and compressors.
Rotary machines such as steam and gas turbines used for power generation and mechanical drive applications, aircraft engines used for propulsion, and compressors used for pressurization, are generally large machines consisting of multiple turbine and compressor stages. In such machines, pressurized fluid flowing through the turbine and/or compressor stages passes through a series of stationary and rotary components. In a typical steam turbine, the stationary components may include the machine casing and packing head, and the rotary component is the rotor. Annular seals mounted on the stationary components are used to control leakage of fluid along the path between the stationary and rotary components. In fact, the efficiency of the turbine is directly dependent on the ability of these seals to prevent such leakage. These seals can be radial or axial in orientation, and can be one of several kinds such as labyrinth packing seals, leaf seals, abradable seals, compliant plate seals, etc. Radial seals are often segmented for assembly reasons and/or for displacement in the radial direction. While radial segmented labyrinth seals have proved to be quite reliable in steam turbines, their performance degrades over time as a result of transient events in which the stationary and rotary components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance. On the other hand, near-contact seals such as brush seals and compliant plate seals can possibly contact the rotor during transient events leading to wear and heat generation. Heat generation, in turn, can cause rotordynamic instabilities, which are detrimental to the machine's operation and performance.
One means of reducing the negative effects of rubs or contact during transient events has been to employ the variable clearance “positive-pressure” (VCPP) arrangement, in which springs are used to hold the seal segments open at a large running clearance under the no- or low-flow transient conditions, when such rubbing is most likely to occur. During steady-state conditions, when the machine is typically operating at a higher load with higher fluid pressures, the ambient pressure around the seal segment overcomes the spring force acting to close the rings to a close running clearance. Examples of known variable clearance positive-pressure (VCPP) labyrinth seals may be found in U.S. Pat. Nos. 6,695,316; 6,022,027; 5,810,365; 5,603,510; 5,002,288; and 4,443,311.
However, the variable clearance positive-pressure arrangement employs segmented seals that respond solely to the machine load. Once the machine reaches a design load, the packing ring segments close and remain closed until the machine load, and therefore the fluid pressure inside the machine, drops adequately. Thermal transients may persist, however, even after the design load has been reached. Therefore, it is ideally desired that the seal segments remain open until the thermal transients subside. Furthermore, the VCPP seals are susceptible to rubbing in case of rotor vibrations during steady-state operation, when the seal segments are forced close by the ambient fluid pressure. In such circumstances, the current VCCP arrangement is not effective in avoiding rubs since it is a passive method for positioning the seal segments. It would be desirable to provide an “actively controlled” seal positioning arrangement in which the seal segments are held open not just during no- or low-flow conditions, which correspond to the start-up and shut-down transients, but can be opened at any other operating condition, when rubbing might occur, and for any desired period of time. During the operating conditions under which rubbing is unlikely, such as steady-state full or partial load machine operation in the absence of rotor vibration, it would be desirable to ‘actively’ move the segments radially inward to their close running clearance. This would enable optimal sealing between the stationary and rotary components during steady-state operation, sustained over the machine life.
So-called “Smart Seals” employ high force capacity pneumatic actuators to provide a radially outward force to “actively” open the seal segments under any machine operating condition. The need to preserve actuator life, however, requires pressure balancing that is achieved by means of pressure control systems. Also, the actuators must be externally pressurized to overcome the fluid ambient pressure, which necessitates an external high-pressure gas supply system. Examples of “Smart Seals” configurations may be found in U.S. Pat. Nos. 6,786,487; 6,655,696; 6,572,115 and 6,502,823.